Epoxy resins are thermoset plastics for coatings, adhesives, structural materials, electrical insulation, encapsulants, etc. Epoxy resins are particularly well suited to protect electronic devices in packaging. Exportes are applied in unreacted or partly reacted form, which means that the viscosity of the material can be quite low, providing ease of processing and good wetting of device surfaces. Curing of the material at some moderate temperature then generates the rigid epoxy matrix desired for device protection. Filler additives are generally included in the matrix to reduce the coefficient of thermal expansion (CTE) of the cured material to minimize stresses induced by the difference in expansion of the plastic and the device during thermal cycling. As described in Lee, H., and Neville, K., Handbook of Epoxy Resins, McGraw Hill (1967), fully cured epoxy resins, with or without filler, are heavily crosslinked insoluble network polymers. As thermosetting compositions, the epoxy materials are difficult or impossible to remove after curing. As a result, removal of such epoxy materials from electronic packages without damage to the circuitry or devices has been and is virtually impossible.
Epoxy encapsulants are particularly valuable for reinforcement of solder joints against thermal fatigue and encapsulation of wire bonded chips. For these applications, stability to ambient moisture is essential because degradation of the encapsulant cannot be tolerated. A severe limitation of the epoxy reinforcement for the solder joints and encapsulation of wire bonded chips, however, is the fact that cured epoxy resins are insoluble and infusible, which means that the reinforced solder joints and wire bonds cannot be reworked. The inability to replace one defective component on a microelectronic assembly renders all the other valuable components on that assembly useless, thus, the non-reworkability of conventional epoxy materials is a severe limitation on their applicability for solder reinforcement or encapsulation of wire bonds.
Another attribute of the epoxy thermosets is their intractability after curing. Curing converts the epoxy thermosets from low molecular weight precursors to a network polymer of essentially infinite molecular weight. Previously considered an asset, the intractability of thermosets has become a liability because of concerns about their longevity in the environment. Many manufacturers are either voluntarily or by government regulation taking responsibility for disposing or recycling their products. Intractable thermosets are not compatible with the concept of design for disassembly and recycling, whether the epoxies are used as structural components, adhesives, or encapsulants. As demand increases for recyclable products, thermosets designed for disassembly on the molecular scale having cleavable diepoxide materials may well offer a means of maintaining the utility of thermoset materials.
U.S. Pat. No. 3,023,174 to Batzer et al. and British Patent 865,340 disclose compositions based on diepoxides having linear ketal or acetal linkages. There is no mention of the possible utility of such diepoxides with respect to cleavability in dilute acid as disclosed in the present invention. In fact, stability of the cured ketal diepoxide in strong acid is specifically mentioned. U.S. Pat. No. 2,896,462 discloses diepoxides containing cyclic acetal groups, which although expected to be sensitive to degradation by acids, are in fact surprisingly resistant to acids.
U.S. Pat. No. 4,153,586 refers to reaction products of epoxides which are not suitable for curing to rigid matrices by reaction with crosslinking agents such as cyclic anhydrides. Although this patent discloses ketal and acetal diepoxides there is no mention of cleavability of the epoxides or their utility as a removable cured epoxy material.
U.S. Pat. Nos. 3,759,954; 3,879,422 and 3,956,317 all disclose compositions of matter covering diepoxides containing one or more cyclic acetals and ketals but there is no mention of cleavability of the epoxides nor of their utility in epoxy compositions that are removable after curing. The compounds are cured to epoxy matrices which are not cleavable.
U.S. Pat. No. 4,159,221, owned by the assignee herein, discloses a method to hermetically seal an electronic circuit package. The reference discloses that the sealant used is an epoxy which is asserted to be readily soluble after being cured. A partially cured epoxy, such as is disclosed in the reference, may be soluble but will not possess the physical properties needed to reinforce solder interconnections and/or wirebonds.